Method of fabricating NAND-type flash EEPROM without field oxide isolation

ABSTRACT

Methods are described for fabricating NAND-type EEPROMs without field oxide isolation. P+ implantations are employed to isolate adjacent memory cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor fabricationmethods and, more particularly, to fabrication of NAND-type flashEEPROMS without field oxide isolation.

2. Description of Related Art

A non-volatile semiconductor memory device is designed to maintainprogrammed information even in the absence of electrical power. Readonly memory (ROM) is a non-volatile memory commonly used in electronicequipment such as microprocessor-based digital electronic equipment andportable electronic devices.

ROM devices typically include multiple memory cell arrays. Each memorycell array may be visualized as including intersecting word lines andbit lines. Each word and bit line (or bit line pair) intersection cancorrespond to one bit of memory. In mask programmable metal oxidesemiconductor (MOS) ROM devices, the presence or absence of an activeMOS transistor at word and bit line intersections distinguishes betweena stored logic ‘0’ and logic ‘1’.

A programmable read only memory (PROM) is similar to mask programmableROM except that a user may store data values (i.e., program the PROM)using a PROM programmer. A PROM device is typically manufactured withfusible links at all word and bit line intersections. This correspondsto having all bits at a particular logic value, typically logic ‘1’. ThePROM programmer is used to set desired bits to the opposite logic value,typically by applying a high voltage that vaporizes the fusible linkscorresponding to the desired bits. A typical PROM device can only beprogrammed once.

An erasable programmable read only memory (EPROM) is programmable like aPROM, but can also be erased (e.g., to an all logic ‘1’s state) byexposing it to ultraviolet light. A typical EPROM device has a floatinggate MOS transistor at word and bit line intersections. Each MOStransistor has two gates: a floating gate and a non-floating or controlgate. The floating gate is not electrically connected to any conductor,and is surrounded by a high impedance insulating material. To programthe EPROM device, a high voltage is applied to the non-floating gate ateach bit location where a logic value (e.g., a logic ‘0’) is to bestored. This causes a breakdown in the insulating material and allows anegative charge to accumulate on the floating gate. When the highvoltage is removed, the negative charge remains on the floating gate.During subsequent read operations, the negative charge prevents the MOStransistor from forming a low resistance channel between a drain bitline and a source bit line (i.e., from turning on) when the transistoris selected.

An EPROM integrated circuit is normally housed in a package having aquartz lid, and the EPROM is erased by exposing the EPROM integratedcircuit to ultraviolet light passed through the quartz lid. Theinsulating material surrounding the floating gates becomes slightlyconductive when exposed to the ultraviolet light, allowing theaccumulated negative charges on the floating gates to dissipate.

A typical electrically erasable programmable read only memory (EEPROM)device is similar to an EPROM device except that individual stored bitsmay be erased electrically. The floating gates in an EEPROM device aresurrounded by a much thinner insulating layer, and accumulated negativecharges on the floating gates can be dissipated by applying a voltagehaving a polarity opposite that of the programming voltage to thenon-floating gates.

A relatively recent development in non-volatile memory is localizedtrapped charge devices. While these devices are sometimes referred to asnitride read only memory (NROM) devices, the acronym “NROM” is a part ofa combination trademark of Saifun Semiconductors Ltd. (Netanya, Israel).

EEPROM arrays can be fabricated in either NOR or NAND configurations.The NAND configuration, which typically comprises parallel strings ofmemory cells connected in series, source-to-drain (NAND strings), may bepreferred over the NOR configuration because of economy in the use ofsemiconductor real estate. Fabrication of NAND-type EEPROM arraysnormally requires that isolation be provided between NAND strings inorder that potentials applied to program a given cell do not influencethe program state of neighboring EEPROM cells. Field oxide formed in asubstrate between NAND strings may be used to provide the neededisolation. Shallow trench isolation (STI) may be employed as well. Fieldoxide and STI normally are formed before cell threshold voltageadjustment can be done. Including field oxide isolation in the design ofEEPROM arrays may contribute undesirably to an increase in the thicknessof individual cells, making it relatively difficult to fabricate flatarrays. Both STI and field oxide may consume substrate area that wouldbe better utilized to provide a greater number of memory cells. That is,the requirement for field oxide isolation regions or STI in EEPROMarrays wastes semiconductor area and contributes to a decrease in thedensity of EEPROM arrays.

A need thus exists in the prior art for an EEPROM array without fieldoxide isolation regions. A further need exists for an EEPROM arraywithout shallow trench isolation.

SUMMARY OF THE INVENTION

The present invention addresses these needs by providing methods forfabricating a NAND-type flash EEPROM without field oxide isolation. Theinvention herein disclosed provides, in an exemplary embodiment, asubstrate doped with a first impurity type in which an implantation ofthe first impurity type is performed in order to control a desiredthreshold voltage of a memory cell. A tunnel oxide layer is formed onthe substrate, a first floating gate layer is deposited on the tunneloxide layer, and a first isolation layer is deposited on the firstfloating gate layer. A plurality of parallel regions is then etched inthe first isolation layer and the first floating gate layer, theparallel regions being oriented in a reference direction and exposingportions of the tunnel oxide layer. A dopant of the first impurity typeis then implanted into the substrate under the exposed portions of thetunnel oxide layer, forming a plurality of parallel electrical isolationregions in the substrate. A second isolation layer may be deposited tooverlie the first isolation layer and the exposed portions of the tunneloxide layer. A portion of the second isolation layer that overlies thefirst isolation layer then may be removed, and the first isolation layerthen removed as well. A second floating gate layer may be deposited onthe first floating gate layer, and a portion of the second floating gatelayer that overlies the second isolation layer may be removed to exposea portion of the second isolation layer. An oxide-nitride-oxide (ONO)layer may then be formed on the second floating gate layer and theexposed portion of the second isolation layer. A layer of conductingmaterial may be deposited on the ONO layer to form a control gate layer.After a re-oxidation step, a plurality of parallel regions of thecontrol gate layer, the ONO layer, the second floating gate layer, thefirst floating gate layer, and the second isolation layer is etched toexpose a plurality of parallel regions of the tunnel oxide layer. Theparallel regions of the tunnel oxide layer are oriented in a directionsubstantially at a right angle to the reference direction. A dopant of asecond impurity type is then implanted into the substrate under theexposed portions of the tunnel oxide layer to form a plurality ofsource/drain regions. In an exemplary embodiment, a dopant of the firstimpurity type may be a p-type dopant, and a dopant of the secondimpurity type may be an n-type dopant.

While the apparatus and method has or will be described for the sake ofgrammatical fluidity with functional explanations, it is to be expresslyunderstood that the claims, unless expressly formulated under 35 U.S.C.112, are not to be construed as necessarily limited in any way by theconstruction of “means” or “steps” limitations, but are to be accordedthe full scope of the meaning and equivalents of the definition providedby the claims under the judicial doctrine of equivalents, and in thecase where the claims are expressly formulated under 35 U.S.C. 112 areto be accorded full statutory equivalents under 35 U.S.C. 112.

Any feature or combination of features described herein are includedwithin the scope of the present invention provided that the featuresincluded in any such combination are not mutually inconsistent as willbe apparent from the context, this specification, and the knowledge ofone skilled in the art. For purposes of summarizing the presentinvention, certain aspects, advantages and novel features of the presentinvention are described herein. Of course, it is to be understood thatnot necessarily all such aspects, advantages or features will beembodied in any particular embodiment of the present invention.Additional advantages and aspects of the present invention are apparentin the following detailed description and claims that follow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow diagram that outlines fabrication of a floating gateEEPROM array according to an exemplary implementation of the method ofthe present invention;

FIGS. 2-11 are cross-sectional views that illustrate steps of theimplementation of the method described in FIG. 1;

FIGS. 12-16 are alternative cross-sectional views illustratingadditional steps of the implementation of the method described in FIG.1;

FIG. 17A is a plan view of an embodiment of a floating gate EEPROM arrayfabricated according to the present invention;

FIG. 17B is a schematic equivalent of the embodiment of the floatinggate EEPROM array illustrated in FIG. 17A;

FIG. 18 is a flow diagram that outlines fabrication of a charge-trappingEEPROM array according to another exemplary implementation of the methodof the present invention;

FIGS. 19-25 are cross-sectional views that illustrate steps of theimplementation of the method described in FIG. 14;

FIGS. 26-31 are alternative cross-sectional views illustratingadditional steps of the implementation of the method described in FIG.18;

FIG. 32A is a plan view of an embodiment of a localized trapped chargeEEPROM array fabricated according to the present invention; and

FIG. 32B is a schematic equivalent of the embodiment of the localizedtrapped charge EEPROM array fabricated according to the presentinvention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same or similar referencenumbers are used in the drawings and the description to refer to thesame or like parts. It should be noted that the drawings are insimplified form and are not to precise scale. In reference to thedisclosure herein, for purposes of convenience and clarity only,directional terms, such as, top, bottom, left, right, up, down, over,above, below, beneath, rear, and front, are used with respect to theaccompanying drawings. Such directional terms should not be construed tolimit the scope of the invention in any manner.

Although the disclosure herein refers to certain illustratedembodiments, it is to be understood that these embodiments are presentedby way of example and not by way of limitation. The intent of thefollowing detailed description, although discussing exemplaryembodiments, is to be construed to cover all modifications,alternatives, and equivalents of the embodiments as may fall within thespirit and scope of the invention as defined by the appended claims. Itis to be understood and appreciated that the process steps andstructures described herein do not cover a complete process flow for themanufacture of NAND-type EEPROM arrays. The present invention may bepracticed in conjunction with various integrated circuit fabricationtechniques that are conventionally used in the art, and only so much ofthe commonly practiced process steps are included herein as arenecessary to provide an understanding of the present invention.

Referring more particularly to the drawings, FIG. 1 is a flow diagramthat summarizes aspects of the present invention as applied to thefabrication of a NAND-type floating gate flash EEPROM array. Steps ofthe method summarized in FIG. 1 will now be described with reference toFIGS. 2-17A. Referring specifically to FIG. 2, a substrate 200 isprovided at step 100. In a typical embodiment, the substrate 200 isformed of silicon that has been doped with a first impurity type. Animplantation 202, also of the first impurity type, may be performed atstep 105 for controlling a threshold voltage of memory cells to beformed on the substrate. The first impurity type, according to anexemplary embodiment, may comprise p-type material such as boron orindium. In an alternative embodiment, the first impurity type maycomprise n-type material such as phosphorous or antimony. In theimplementation described herein, the first impurity type comprisesp-type material. A thin (typically 80 Å to 100 Å) tunnel oxide layer205, which may comprise silicon dioxide, may be formed on the substrateat step 110. A first floating gate layer 210 and a first isolation layer215 may be deposited at step 115. The first floating gate layer 210 maybe formed of a film of n+ doped or undoped polysilicon. The firstisolation layer 215 typically is formed of silicon nitride. The firstisolation layer 215 then may be coated with photoresist, which may bepatterned to define parallel regions in a reference direction, which maycoincide with a bit line direction. The first isolation layer 215 andthe first floating gate layer 210 then may be etched at step 120 to forma plurality of parallel regions 217 as illustrated in FIG. 3. Theetching removes portions of the first isolation layer 215 and the firstfloating gate layer 210, thereby forming first floating gates 211. Theparallel regions 217 are oriented in the reference direction (i.e.,perpendicular to the plane of the page of the diagram of FIG. 3).

The etch process that forms the plurality of parallel regions 217 mayinclude, for example, a two-step etching process performed in sequence.The first etch process may be a selective etch process (e.g., a dryplasma etch process) in which the etchant has a higher selectivity fornitride than for photoresist. The first etch process may remove materialin the first isolation layer 215 (that may be formed of siliconnitride), using photoresist as a mask. A second etch process may be aselective etch process (e.g., a dry plasma etch process) in which theetchant has a higher selectivity for silicon than for photoresist. Thesecond etch process may remove material in the first floating gate layer210 (that may be formed of polysilicon), likewise using photoresist as amask. Remaining photoresist material then may be removed.

Formation of the plurality of parallel regions 217 exposes the thintunnel oxide layer 205 in those regions. A first impurity type dopant,which in the illustrated embodiment is a p-type dopant, may then beimplanted through the exposed portions, i.e. strips, of the thin tunneloxide layer 205 into the substrate 200 at step 125. This implantation,which is self-aligned to the strips of the exposed tunnel oxide layer205, forms p-type electrical isolation regions. It should be noted thatthe impurity type of the isolation regions is opposite to that of thedrain/source regions described infra. According to an illustrativeembodiment depicted in FIG. 4, the implantation of the p-type dopant iscarried out using a two-step process. In the first step, alightly-concentrated p-type dopant, such as boron or indium, isimplanted to form a p-implantation 220. Then a relatively thin spacerlayer 219, which may be formed of, for example, silicon dioxide orsilicon nitride, is deposited or thermally grown on the tunnel oxidelayer 205 and on sidewalls of the plurality of parallel regions 217. Asecond, heavier implantation, with higher concentration of a p-typedopant, such as boron or indium, i.e. a p+ implantation 221, is thenperformed to form p+ isolation regions 222. The p− implantation 220 canprovide suitable isolation between NAND strings and can provide a higherjunction breakdown between a plurality of source/drain regions(typically implemented by n+ implantations as described below) and theplurality of p+ isolation regions 222. This higher junction breakdownmay prevent junction leakage during programming of memory cells. In amodified embodiment, only a single implantation of the first impuritytype (the p+ implantation 221 in the present instance) may beimplemented.

At step 130 a second isolation layer 225 is deposited, covering thefirst isolation layer 215 and the exposed portion of tunnel oxide layer205 in the plurality of parallel regions 217 as shown in FIG. 5.According to an illustrative embodiment, the second isolation layer 225comprises high density plasma (HDP) oxide. A portion of the secondisolation layer 225 is then removed at step 135. An exemplary method ofremoval of the portion of the second isolation layer 225 comprisesetching back by performing an oxide dip sufficient to expose a surfaceof the first isolation layer 215, the result of which is illustrated inFIG. 6. Chemical mechanical polishing (CMP) may be employed on thestructure of FIG. 5 in another embodiment to remove a portion of thesecond isolation layer 225 and expose a surface of the first isolationlayer 215. Subsequently, the first isolation layer 215 may be removed atstep 140 using for example a lift-off process in which a wet etch isperformed using an etchant having a greater selectivity for the firstisolation layer 215 than for the second isolation layer 225, resultingin the structure shown in FIG. 7.

With reference to FIG. 8, a second floating gate layer 230, which may beformed of a film of doped or undoped polysilicon, is then deposited overthe first floating gates 211 and the remaining second isolation layer225 at step 145. The second floating gate layer 230 is then covered witha patterned photoresist and etched to remove a portion 232 (FIG. 9) ofthe second floating gate layer 230 disposed above the second isolationlayer 225 at step 150. The removal of portion 232 forms second floatinggates 231 as shown in FIG. 9. The second floating gates 231 areelectrically connected to and extend beyond the edges of the firstfloating gates 211. Together, the first floating gates 211 and secondfloating gates 231 form composite floating gates having an increasedeffective size and gate coupling ratio compared to the size and gatecoupling ratio of the first floating gates 211. This increased effectivesize and gate coupling ratio may improve the performance of theresulting memory cells.

An interlayer dielectric, which as presently embodied may comprise anoxide-nitride-oxide (QNO) layer 235 as shown in FIG. 10, is then formedabove the second floating gates 231 and the exposed portion of thesecond isolation layer 225 at step 155. Typically, formation of the ONOlayer 235 comprises depositing a layer of oxide material, which maycomprise silicon dioxide. A layer of nitride material, which maycomprise silicon nitride, is deposited over the oxide, and another oxidelayer, which also may comprise silicon dioxide, is formed over thenitride layer. A layer of conducting material, which may be formed ofpolysilicon, then is deposited on the ONO layer 235 at step 160 to forma control gate layer 240 as shown in FIG. 11. The ONO layer 235 operatesas an isolation dielectric between the control gate layer 240 and thefloating gates formed by the second floating gates 231 and the firstfloating gates 211. A re-oxidation process may be performed to removeetching damage.

The preceding cross-sectional views of FIGS. 2-11 are taken along a linecorresponding to line A-A′ in FIG. 17A. The following cross-sectionalviews of FIGS. 12-15A are taken along a line corresponding to line B-B′nominally perpendicular to line A-A′ in FIG. 17A.

The cross-sectional depiction of FIG. 12 shows the result of performingsteps 100-160 of the exemplary method described above when viewed alongthe line B-B′. Following formation of the structure of FIGS. 11 and 12,the control gate layer 240 may be covered with patterned photoresist andetched at step 165 to form a plurality of parallel regions 242 thatexpose the tunnel oxide layer 205 as shown in FIG. 13. The parallelregions 242 are oriented in a direction substantially perpendicular tothe reference direction. The etch process that forms the parallelregions 242 may comprise, for example, a multi-step etch process inwhich each step of the etch process is a selective etch process thatremoves one or more layers of material from the parallel regions 242.For example, the first etch process may be a selective etch process inwhich the etchant has a higher selectivity for silicon than forphotoresist. The first etch process may remove material in the controlgate layer 240, thereby forming word lines 241 described more fullyinfra with reference to FIGS. 17A and 17B. A second etch process may bea selective etch process in which the etchant has a higher selectivityfor oxide than for photoresist. The second etch process may removematerial in an upper oxide layer of the ONO layer 235. Similarsuccessive etch processes may be used to remove portions of theremainder of the ONO layer 235, the second floating gates 231, thesecond isolation layer 225, and the first floating gates 211 until thetunnel oxide layer 205 is exposed.

With the tunnel oxide layer 205 exposed, a second impurity typeimplantation, which as presently embodied comprises an n-typeimplantation, may be performed at step 170 to form a plurality ofsource/drain regions 247 in the substrate 200 under the exposed portionsof the tunnel oxide layer 205 as shown in FIG. 14. An embodiment of then-type implantation step, which may implant an n-type material such asphosphorous or antimony, may comprise a two-step implantation process.According to this embodiment, a lightly-concentrated n-typeimplantation, i.e. an n− implantation 245, is performed after formationof the parallel regions 242. The first implantation step, i.e., the n−implantation 245, may act to reduce junction leakage. Following the n−implantation step, a thin silicon nitride or oxide film or layer may begrown or deposited to form a relatively thin spacer (e.g., siliconnitride or oxide) layer 243. Then a heavy n-type implantation, i.e. ann+ implantation, is performed. The n+ implantation step may act toreduce resistance of the source/drain regions 246. Interlayer dielectricmaterial 250 may be deposited over the structure at the conclusion ofthe aforementioned steps of the method as shown in FIG. 15A. Anothercross-section, taken along line D-D′ of FIG. 17A is illustrated in FIG.15B, wherein the regions 245′ and 246′ comprise the n− and n+implantations, respectively, formed over the p+ isolation regions 222(FIG. 11). In a modified embodiment, only a single implantation, e.g.the n+ implantation 246, of the second impurity type may be implemented.FIG. 16 illustrates yet another cross-section taken along line C-C′ ofFIG. 17A.

FIG. 17A is a plan view of an exemplary result of performing theabove-described steps. The figure illustrates word lines 241 andsource/drain regions 247. A schematic diagram of the circuit equivalentof the structure illustrated in FIG. 17A is shown in FIG. 17B.Horizontal structures labeled WL in FIG. 17A represent control gates orword lines 241. Vertical structures labeled BL representseries-connected transistors that share sources and drains designated inthe diagram as n+ 247 (cf. FIG. 15A). That is, these vertical structurescomprise NAND strings. Individual memory cells, which also comprisefloating gates, generally reside at the intersections of word lines WL241 and pairs of bit lines BL. Isolation between NAND strings isprovided by the regions designated as p+ 222 (cf. FIG. 11) and by p+implanted areas between vertical pairs of implanted regions p+ 222 (cf.FIG. 16). No shallow trench isolation (STI) or field oxide isolation isrequired in this implementation. These features may enhance thesuitability of the structures described herein to be used in highdensity flash EEPROM devices. Additionally, the tunnel oxide layer 205has no STI edge or field oxide edge, so better reliability of memorydevices can be achieved by applying the method of the present invention.

An alternative embodiment of a NAND-type EEPROM array, i.e. asilicon-ONO-silicon (SONOS) NAND flash EEPROM array, employing localizedtrapped charge storage is illustrated in FIGS. 18-32B. Elements havingreference designators of the form 4 xx in FIGS. 18-32B have a meaningcorresponding to reference designators of the form 2 xx in FIGS. 1-17Bunless indicated otherwise. For instance, reference designators 420,421, 425, and 443 in FIGS. 18-32B have descriptions that may be the sameas, respectively, 220, 221, 225, and 243 in FIGS. 1-17B. An ONO layermay be used to store localized trapped charge in the embodimentdescribed. The fabrication of this alternative embodiment may followsteps similar to those outlined for fabrication of the NAND-typefloating gate EEPROM array as summarized in FIG. 1. An implementation ofthe inventive method for fabricating NAND-type localized trapped chargeEEPROM arrays, which may employ, for example, ONO storage, is outlinedin the flow diagram of FIG. 18. Referring additionally to FIG. 19, whichis a cross-sectional view taken along line E-E′ of FIG. 32A, a p-typesubstrate is provided at step 300, and a p-type implantation 402 isperformed at step 305, steps 300 and 305 being similar to respectivesteps 100 and 105 of FIG. 1. An ONO layer 405 is then formed on thesubstrate 400 at step 310. A layer of conducting material, such aspolysilicon, is deposited to form a first control gate layer 410, and afirst isolation layer 415 is likewise deposited at step 315.

Steps 320, 325, 330, 335, and 340 in the present implementation, andcorresponding FIGS. 20-24, representing cross-sections taken along lineE-E′ of FIG. 32A, may be similar to corresponding steps 120, 125, 130,135, and 140 described supra with reference to FIG. 1 and FIGS. 2-7. Alayer of conducting material, which may be polysilicon in arepresentative embodiment, may be deposited at step 360 to form a secondcontrol gate layer 440 as illustrated in FIG. 25, which also is across-sectional view taken along line E-E′ of FIG. 32A.

The cross-sectional view illustrated in FIG. 26 is taken along line F-F′of FIG. 32A, as are the views of FIGS. 27-30. Continuing with themethod, a plurality of parallel regions 442 shown in FIG. 27 is thenformed at step 365 by forming a patterned photoresist over and etchingthe structure of FIG. 26. Creating the regions 442 forms control gates411 from the first control gate layer 410 and word lines 441 from thesecond control gate layer 440. The etch process that forms the parallelregions 442 may include a two-step etch process analogous to thatdescribed above for forming the parallel regions 242 (FIG. 13). Ann-type implantation 445, 446 may then be performed at step 370 to formsource/drain regions 447. A two-step process similar to that describedfor forming the n-type source/drain regions 247 at step 170 of theearlier described method may be employed. An interlayer dielectric 450then may be deposited to yield the structure of FIG. 29. Anothercross-sectional view of this localized trapped charge embodiment of theNAND-type EEPROM array taken along the line H-H′ of FIG. 32A isillustrated in FIG. 30. The areas designated as 445′ and 446′ comprisen− and n+ implantations, respectively, formed over the p+ isolationregions 422 (FIG. 25). FIG. 31 is a cross-section of the localizedtrapped charge NAND-type EEPROM array taken along line G-G′ in FIG. 32A.FIG. 32A is a plan view of an exemplary result of performing the methodsteps summarized in FIG. 18. A schematic diagram of the circuitequivalent of the structure illustrated in FIG. 32A is shown in FIG.32B.

In view of the foregoing, it will be understood by those skilled in theart that the methods of the present invention can facilitate formationof memory devices, and in particular memory devices not having fieldoxide, in an integrated circuit. The above-described embodiments havebeen provided by way of example, and the present invention is notlimited to these examples. Multiple variations and modification to thedisclosed embodiments will occur, to the extent not mutually exclusive,to those skilled in the art upon consideration of the foregoingdescription. Additionally, other combinations, omissions, substitutionsand modifications will be apparent to the skilled artisan in view of thedisclosure herein. Accordingly, the present invention is not intended tobe limited by the disclosed embodiments, but is to be defined byreference to the appended claims.

1. A method of fabricating an NAND-type flash EEPROM, comprising;providing a substrate doped with a first impurity type; forming a tunneloxide layer on the substrate; depositing a first floating gate layer onthe tunnel oxide layer; depositing a first isolation layer on the firstfloating gate layer; etching portion of the first isolation layer andportion of the first floating gate layer to form a plurality of parallelregions for exposing portions of the tunnel oxide layer, the parallelregions being oriented in a reference direction; implanting a dopant ofthe first impurity type through the exposed portion of tunnel oxidelayer into the substrate, thereby forming a plurality of parallelelectrical isolation regions in the substrate; and depositing a secondisolation layer to cover the first isolation layer and the exposedportions of the tunnel oxide layer.
 2. The method as set forth in claim1, further comprising: removing a portion of the second isolation layerthat overlies the first isolation layer; removing the first isolationlayer; depositing a second floating gate layer on the first floatinggate layer and the remaining second isolation layer; and removing aportion of the second floating gate layer that overlies the secondisolation layer, thereby exposing a portion of the second isolationlayer.
 3. The method as set forth in claim 2, further comprising:forming an oxide-nitride-oxide (ONO) layer on the second floating gatelayer and the exposed portion of the second isolation layer; depositinga layer of conducting material on the ONO layer to form a control gatelayer; and performing a re-oxidation process on the layer of conductingmaterial.
 4. The method as set forth in claim 1, further comprisingimplanting a dopant of a second impurity type into the substrate underthe exposed portions of the tunnel oxide layer, thereby forming aplurality of source/drain regions.
 5. The method as set forth in claim4, wherein the implanting of a dopant of the first impurity typecomprise: performing a light concentration implanting; forming arelatively thin spacer layer; and performing a heavy implantation;wherein the light concentration implantation acts to increase a junctionbreakdown between the plurality of source/drain regions and theplurality of isolation regions, thereby preventing junction breakdownwhile programming and reducing junction leakage.
 6. The method as setforth in claim 4, wherein the implanting of a dopant of the secondimpurity type comprises: performing a light concentration implantationafter the etching of a plurality of parallel regions in the layer ofconducting material, the light concentration implantation acting toreduce junction leajage; depositing a relatively thin spacer material;and performing a heavy implantation, the heavy implantation acting toreduce source/drain resistance.
 7. The method as set forth in claim 1,wherein the providing of a substrate is followed by performing animplantation of the first impurity type into the substrate, theimplantation being controlled according to a desired threshold voltage.8. The method as set forth in claim 1, wherein: the depositing of asecond isolation layer comprises depositing high density plasma oxide;and the depositing of a second floating gate layer comprises depositingone of a film of doped polysilicon and a film of undoped polysilicon. 9.The method as set forth in claim 3, wherein the depositing of a layer ofconducting material comprises depositing a layer of polysilicon, andwherein the method further comprises etching a plurality of parallelregions of the control gate layer, the ONO layer, the second floatinggate layer, the first floating gate layer, and the second isolationlayer to expose a plurality of parallel regions of the tunnel oxidelayer, the exposed plurality of parallel regions of the tunnel oxidelayer being oriented in a direction substantially at a right angle tothe reference direction.